1. Field of the Invention
The present invention relates to a variable valve device and more specifically to a variable valve device used to change the displacement (generally called “lift degree”) of an intake valve in an engine.
2. Description of the Background Art
JP 2003-41976 A discloses a device that detects the operation angle of a control shaft in a variable valve mechanism that continuously changes the valve lift degree and the valve operation angle of an intake valve in an internal combustion engine in response to the operation angle of the control shaft. The device includes an operation angle sensor that generates an output in response to the operation angle of the control shaft, such that an intake air amount is estimated based on the operation angle detected based on the output.
However, when the operation angle sensor (that corresponds to the “control shaft sensor” according to the present invention) has a failure, the intake air amount can no longer be estimated, so that the fuel injection amount or the like cannot be controlled.
In order to solve this problem, there is a method of detecting an intake air amount using an airflow sensor in an intake pipe. However, when the engine speed increases, the overlap state in which the intake valve and the exhaust valve are both open is prolonged, and exhaust gas is discharged into the intake pipe through the intake valve, (in other words, spitback is caused) which lowers the detection precision for the intake air amount. Motorcycles in particular are driven at high engine speed more often than automobiles and accordingly, in motorcycles, this problem cannot be ignored.
In addition, a motorcycle vibrates more than an automobile and an airflow sensor could be more prone to failures depending on where the sensor is provided. A motorcycle has a limited installation space as compared to the automobile, and therefore it is difficult to secure a location to provide such an airflow sensor.
A pressure sensor may be provided in an intake pipe and the difference in the intake pressure may be detected to estimate the intake air amount. However, there is little difference in the intake pressure in motorcycles without throttle valves, and this method cannot be applied to this kind of motorcycle. Furthermore, in a motorcycle with a throttle valve, the difference in the intake valve changes as the valve lift degree changes, and therefore it is difficult to accurately estimate the intake air amount.
Two control shaft sensors may be provided to a control shaft, but this increases the manufacturing cost. If two sensors are provided, additional control to determine which sensor has a failure must be carried out.